In general, an on-board evaporative emission control system for an automotive vehicle comprises a vapor accumulating canister which serves as a collector of fuel vapors from the headspace of the fuel tank, and a purge valve which discharges on demand the fuel vapor-air mixture from the canister into an intake manifold of the engine in a controlled manner. The purge valve comprises an actuator, generally a solenoid, which acts upon a valve, generally of diaphragm or poppet type. The actuator is controlled by using a pulse-width modulation or other methods in order to regulate the flow of the fuel vapor-air mixture through the valve in a proportional matter.
These solenoid systems gain proportional flow by turning the valve on/off at low frequencies, creating an undesirable noise and providing rudimentary flow control. Some systems use a piezoelectric actuator. One such system is a piezoelectric actuator with a hydraulic amplifier as a substitute to the solenoid actuator, used to decrease the response time of the purge valve. A hydraulic system is used as a mechanical amplifier in order to increase the limited travel distance of the piezoelectric actuator.
Despite some response time improvement, this system introduces additional complexity due to the hydraulic system, which potentially decreases the reliability of the valve. With maximum open purge valve the flow is determined by the differential pressure between the input pressure (generally atmospheric pressure in the canister) and output pressure (the vacuum created in the intake manifold). While the input pressure does not change substantially, the differential pressure changes as the vacuum created in the intake manifold changes. As a result, the current state of the art EVAP systems have some substantial disadvantages. In a given purge valve the flow is determined primarily by the vacuum in the intake manifold. This limits the ability of the EVAP systems to always respond adequately to the needs of the engine. Hence the system does not control optimally in situations where the vacuum in the intake manifold is low and high flow is still desired, e.g. low RPM or turbo-charged engines.
Gas (air) pumps have been used as part of EVAP systems. In one application a gas pump, internally integrated with a canister, has been used to introduce atmospheric air into the canister to facilitate the flushing of the fuel vapors from the canister. In another application, a gas pump has been used as an actuator for the purge valve. No such system actively draws gas vapors and controls them from the gas tank to meet low or no vacuum situations.
Additional complications related to the use of the conventional purge valve are associated with its use in EVAP systems of engines with boosted power, which use superchargers or turbochargers (forced induction devices). The output of the purge valve is connected to the intake manifold, which in this particular case is the output of the supercharger. In this way, the output of the purge valve (injection point) is exposed to the output high pressure of the supercharger. In conventional purge valves, this decreases substantially the differential pressure between the input and output of the purge valve and either decreases or makes impossible the flow of vapors through the valve, limiting the ability to introduce the vapors when desired. In order to circumvent this problem, an evaporative emission purging system has been used, with an output connected to the intake air, upstream from a forced induction device. In this way, the purge valve uses the vacuum created by the supercharger at its air input to create a bigger differential pressure, which results in a bigger flow. To improvement this device, a venturi tube can be used, positioned in a restricted area upstream from the forced induction device to additionally decrease the pressure at the injection point and to additionally increase the flow through the purge valve. This valve with the venturi tube reduces the requirements on the air pump. Despite these changes described above, the flow in the purge valve depends heavily on the vacuum created at the injection point, i.e. it depends on the engine working status, e.g. RPM.